1. Field of the Invention
This invention relates to a brazing flux composed of a mixture of fluoro-aluminum-complexes including potassium and cesium.
2. Description of the Prior Art
When aluminum or an aluminum-alloy (hereinafter will be merely called an aluminum material) is brazed, it has been usual to employ (as a brazing filler metal) a eutectic alloy of aluminum and silicon having a melting point which is somewhat lower than that of the aluminum material. In order to bond the filler metal to the aluminum material satisfactorily, it is necessary to remove an oxide film from the surface of the aluminum material. For that purpose, a flux is applied to a portion of the material to be brazed together with the filler metal. There has recently come to be used a non-corrosive flux composed of potassium fluoride-aluminum fluoride (KF-AlF.sub.3) complex. This substance melts at the eutectic point of KF-AlF.sub.3 and exhibits an excellent flux action. It is, however, at a temperature of 560.degree. C. or above that this flux begins to melt (U.S. Pat. No. 3,951,328). It is, therefore, necessary to use a brazing filler metal having a melting point which is several tens of degrees higher than 560.degree. C., and employ a correspondingly high brazing temperature. When a torch, such as acetylene flame or the like, is used as a source of heat for brazing, it is difficult to control the brazing temperature if the melting points of the filler metal and flux are high. Therefore, the use of the KF-AlF.sub.3 flux requires a high degree of skill and experience.
The applicant has found that a composite of cesium fluoride-aluminum fluoride (CsF-AlF.sub.3) melts at an extremely low temperature compared with the KF-AlF.sub.3 flux. And moreover, it exhibits an effective flux action for the aluminum material containing magnesium (Mg). These have been filed with titles of the invention of "Brazing Flux" (PCT Application No. JP85/00705) and "A Method of Brazing an Aluminum Material" (Japanese patent application No. 10445/1985), respectively.
This CsF-AlF.sub.3 flux begins to melt at a low temperature, such as 440.degree. C. to 460.degree. C., and, therefore, it is possible to use a filler metal having a melting point which is about 120.degree. C. lower than that of the KF-AlF.sub.3 flux. Consequently, the brazing temperature can be also lowered.
However, when it is necessary to use a filler metal having a high melting point (for example, in case that an aluminum filler metal containing Cu cannot be applied to an aluminum material used in a corrosive environment), the difference between the melting temperature of the flux and the brazing temperature becomes too large so that sublimation and oxidation of the flux are promoted while the material to be brazed is heated to the brazing temperature. And consequently, it has been encountered that most of the flux cannot effectively be used. So, the inventor's investigations have progressed in order to obtain a flux the melting temperature of which can suitably be selected, leaving merits of the CsF flux herein.